Find the sides of the parallelogram.

Well, since it's a parallelogram, you know that you can equal the parallel sides to each other. 

2x+4=10 (subtract 4 from both sides)
2x=6 (divide by 2 on both sides)
x=3. 

Then what you would have to do is plug in 3 for x. 

2(3)+4=? (Multiply 3 and 2)
6+4=? (Add 6 and 4)
10
 The opposite side (in this case parallel) of 10 is also 10. So if you got 10, it is parallel. Now let's work on the other side. 

We still have to plug in 3 for x on both equations this time.

7(3)-1=6(3)+2 (Multiply 7 and 3 and then 6 and 3)
21-1=18+2 (Subtract 1 from 21 and Add 18 and 2)
20=20. 

There are the sides. As long as the parallel sides are equal_and it says that the rhombus is a paralellogram, you have done the math correctly.